ssize_t rio_readn(int fd,void *usrbuf,size_t n)
{
    size_t nleft = 0;
    sszie_t nread;
    char *bufp = usrbuf;

    while(nleft > 0)    
    {
        if((nread = read(fd,bufp,nleft))<0)
        {
            if (errno == EINTR)
                nread = 0;
            else
                return -1;
        }
        else if (nread == 0)
            break;
        nleft -= nread;
        bufp += nread;
    }
    return (n-nleft);
}
